Small-gap light sensor

ABSTRACT

A sensor having a light detector with a small gap between the cathode and anode to enable a high pressure cavity resulting in a long lifetime of the detector due to insignificant sputtering from the cathode and subsequent minimal burying of the noble gas in the cavity. The detector may be made with MEMS technology and its techniques. The sensor may contain an array of light detectors. Some of the detectors may be UV detectors.

BACKGROUND

The invention relates to sensors and particularly to sensors having MEMSstructures. More particularly, the invention relates to light sensorshaving MEMS structures.

SUMMARY

The present invention may be a multi-wafer tube-based light sensorhaving an exceptionally small gap for discharge and a significantly highpressure in the cavity.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side cross-section view of the sensor;

FIG. 2 is a top cross-section view of the sensor; and

FIG. 3 is a graph of the sensor cavity breakdown voltage versus theproduct of the pressure of the cavity and the distance of the cathodeand anode spacing.

DESCRIPTION

The present sensor 10 is a MEMS (micro electro mechanical systems)device fabricated as a light detector. This detector may be used for thedetection of infrared (IR), visible and ultraviolet (UV) light,depending on the materials used in its structure. The illustrativeexample described here may be a UV light detector. It may be used forflame detection and other applications having UV attributes. Related artUV detection tubes are bulky, fragile, and have limited lifetimes. Thepresent MEMS detector may be miniaturized, robust, and have a longlifetime. The present MEMS detector may cost significantly less to buildthan the related art detection tubes. The present detector may be builtwith MEMS assembly techniques. The invention may be regarded as a tubetype device despite its containment in a MEMS solid state enclosure. Itmay have other applications besides UV detection.

A typical UV sensor tube may have relatively large volume, e.g., 9000cubic millimeters (mm³). One of the concerns is the lifetime of therespective tube which may be limited due to the consumption of neon gasin the tube. The consumption of neon may be due to the sputtering of acathode material which buries the neon. Such tube may operate with aspacing of about 500 microns between the anode and cathode in a cavityhaving a pressure of about 100 Torr. The discharge gas composition mayinclude neon with about 15 percent of composition being H₂. Other noblegases besides neon may be used. The addition of H₂ may reduce themetastables which are secondary discharges that may occur during and/orafter a primary discharge between the cathode and anode in the cavity.

The cathode-to-anode spacing may be made around 125 microns in thepresent device. Fabrication with MEMS technology makes such a small gappossible. Minimum gaps of other tubes in the art may be about four timeslarger. Relative to the other UV tubes, the internal cavity pressure maybe raised about four times from, for instance, 100 Torr to about 400Torr. The four-fold pressure increase and the four-fold reduction in thecathode-to-anode gap keep the tube cavity conditions at a Paschen point25 of the same breakdown or discharge voltage, as shown in the graph ofFIG. 3. It may be advisable to have a design that keeps the point within20 percent of the original Paschen point. The sputter rate of a cathodein a device may have a relationship of 1/P⁵ where P is pressure of thegas in the cavity. This relationship may vary among different structuresof the device. However, for an illustrative example, if the pressure isincreased by approximately four times, the sputtering rate of thecathode material may be reduced to 1/1024 of the rate under the samepressure. Further, increasing volume of the cavity makes room for moreneon and consequently may proportionally increase the lifetime of thetube.

FIG. 1 shows a cross-section view of the MEMS tube 10. The base orsubstrate 11 may be fabricated from a fused silica. On the fused silicabase 11 may be a frit 12 formed as a seal between a fused silica spacer13 and base 11. A top 14 may have an anode 15 formed on a bottom surfaceof the top 14. Anode 15 may be situated on a peripheral seal 16 which inturn is situated on the spacer 13. This seal 16 may effectively hold toplayer 14 in place, though with the thin anode layer 15 in between, toform a cavity 26 between a cathode 18 and anode 15 of the tube 10. Theanode may be a metal grid with openings so that light 30 may enterthrough the fused silica top layer 14 and the anode 15 into the cavity.Or anode 15 may be a material that is conductive, and is transparent ortransmissive relative to light 30 to be detected by sensor 10. Depositedor formed on at the center of the substrate or base 11 of tube 10 may bethe cathode 18. Cathode 18 may have a distance 19 from anode 15 whichmay be about 125 microns. Other tube 10 designs may result in othermagnitudes for distance 19. A thickness 20 of cathode 18 composed oftungsten may be about one micron. Thickness 20 may be varied for othercathode materials. Cathode materials may include tungsten, copper,nickel, gold, silver, nickel-iron, barium oxide, cesium, hafnium,molybdenum, and the like. The seal 16 between anode 15 and spacer 13 maybe a Eutectic gold/silicon seal, or seal 16 may be of some appropriateinsulative material.

The cathode 18 material may be selected to provide a long wavelengthlimit of the detector 10 spectral response. The cathode material mayphoto emit electrons below a certain wavelength (i.e., a photo emissionthreshold). The window (i.e., top 14 and anode 15) of the detector 10may provide the short wavelength limit of the detector spectralresponse. However, the window may also have a filter that limits some ofthe long wavelength radiation or light impinging the detector. Thus, thekinds of materials used for the top 14, anode 15 and cathode 18 may beselected to determine the spectral response of the detector.

A trench 17 may be formed around the cathode 18 to add more cavityvolume to cavity 26. A result of the trench 17 may be an island ormesa-like structure that supports the cathode 18. A bridge 27 may beformed across the trench 17. On bridge 27 may be a conductor 28connecting cathode 18 to the periphery of base or substrate 11 forconnection purposes outside of the cavity of tube 10. The peripheralseal 12 may be situated over or formed across conductor 28. On the topof conductor 28 may be formed a thin glass or other insulative coating29 from cathode 18 to the seal 12 to hinder possible shorting from thecathode 18 with anode 14 inside the cavity due to a possibleaccumulation of metal sputter from the cathode 18 during the operationallifetime of the device 10. Spacer 13, situated on seal 12, may likewisehave electrical insulative properties. Thus, cathode 18 and anode 15 maybe connected externally outside of the cavity of tube along with keepingthe cavity hermetically sealed.

Relative to the cathode 18, at standard pressure of 100 Torr, 25 micronsof copper may provide an adequate lifetime, for example, 10,000 hours.For that lifetime, only about one micron of tungsten may be sufficient.Tungsten may be regarded as sputtering less material than nickel, underthe same cavity and electrical conditions, by a factor of about 20.Copper may sputter more than nickel. It is fair to conclude that coppersputters about 25 times greater than tungsten. The sputter rate at ahigher pressure may be reduced by up to R^(n), where “R” is the ratio ofthe pressure increase and “n” is power of R, and as applied with theabove-noted relationship, 4⁵=1024. Thus, the needed thickness for thetungsten cathode may be less than one micron.

Various factors may play a part in the material and thickness of thecathode. For instance, if the sputter rate is reduced by about 1000times, then neon burial may be reduced by about 1000 times due to thefour-fold increase of the cavity pressure to about 400 Torr. Thus, for asimilar lifetime of the tube, which is dependent on the presence of theneon, the required volume for the neon (or other noble gas) may be about1000 times less than the volume of the typical related art tube.

In the case where a typical UV tube may have a volume of about 9000 mm³at a pressure of 100 Torr and an anode-to-cathode distance of about 500microns, the normal lifetime of such tube may be about 10,000 hours. Forthe new and present tube 10, having an increase of pressure to 400 Torrand a decrease of distance or gap between the anode and cathode to about125 microns, the volume of the tube may be reduced by a factor of 1000down to 9 mm³ for a similar lifetime in view of the above-notedinformation.

FIGS. 1 and 2 show the layout of device 10 relative to cavity 26.Without the trench or channel 17, the cavity for the noble gas wouldhave dimensions of approximately 5 mm×5 mm×0.125 mm, resulting in avolume of about 3 mm³. Trench 17 may add more volume to a total tubecavity volume. For instance, looking at the Figures, one may note thedimensions 21, 22, 23 and 24 to be 1, 1, 3 and 6 millimeters (mm),respectively. Thus, trench 17 could add about 16 mm³, i.e., ((5 mm×5mm)−(3 mm×3 mm))×1 mm≈16 mm³. The resultant volume of the tube cavitymay be about 19 mm³. FIGS. 1 and 2 are not necessarily drawn to scale.

Although the invention has been described with respect to at least oneillustrative embodiment, many variations and modifications will becomeapparent to those skilled in the art upon reading the presentspecification. It is therefore the intention that the appended claims beinterpreted as broadly as possible in view of the prior art to includeall such variations and modifications.

1. A light detector comprising: a substrate; a first conductive layerformed on a first surface of the substrate; a top layer; and a secondconductive layer formed on the first surface of the top layer; andwherein: the top layer is attached to the substrate; the firstconductive layer and the second conductive layer are situated in acavity; the top layer is significantly transparent to light; and thesecond conductor is significantly transparent to light.
 2. The detectorof claim 1, further comprising: a trench formed in the substrate; andwherein the trench is contiguous with the cavity.
 3. The detector ofclaim 2, further comprising a noble gas in the cavity.
 4. The lightdetector of claim 2, wherein: the first conductive layer is a cathode;and the second conductive layer is an anode.
 5. The detector of claim 4,wherein: the distance between the cathode and the anode is less than 200microns; and the pressure of the cavity is greater than 250 Torr.
 6. Thedetector of claim 5, wherein the detector is fabricated with MEMStechnology.
 7. The detector of claim 6, wherein: a pressure in thecavity is x; and a distance between the cathode and anode is y; and aproduct of x and y is approximately equal to a Paschen value of adischarge voltage between the cathode and the anode.
 8. The detector ofclaim 7, wherein the Paschen point has a value between 25 and 75Torr-mm.
 9. A light detector comprising: a substrate having a firstconductive layer; a spacer layer situated on a perimeter of thesubstrate; a top layer having first surface situated on the spacerlayer; and a second conductive layer formed on the first surface of thetop layer; and wherein the top layer and the second conductive layer aresignificantly transparent to light.
 10. The detector of claim 9, furthercomprising: a trench formed in the substrate; and wherein the trench anda space between the first and second conductive layers form a cavity.11. The detector of claim 10, wherein the distance between the first andsecond conductive layers is less than 200 microns.
 12. The detector ofclaim 11, wherein the detector is fabricated with MEMS technology. 13.The detector of claim 12, wherein a pressure of a noble gas within thecavity is greater than 250 Torr.
 14. A light detector comprising: asubstrate having a first conductive layer situated on about a centerportion of a first surface of the substrate; a spacer situated on aperimeter of the first surface of the substrate; a top layer havingfirst surface; and a second conductive layer formed on the first surfaceof the top layer; and wherein the second conductive layer and the firstsurface of the top layer are situated on the spacer to form a cavitycontaining the first and second conductive layers.
 15. The detector ofclaim 14, wherein the detector is fabricated with MEMS technology. 16.The detector of claim 14, further comprising a trench formed in thesubstrate between the perimeter of the first surface of the substrateand the first conductive layer on the first surface of the substrate.17. The detector of claim 16, wherein the trench is a part of thecavity.
 18. The detector of claim 17, wherein the top layer and thesecond conductive layer are significantly ultraviolet transparent. 19.The detector of claim 18, further comprising a noble gas in the cavity.20. The detector of claim 19, wherein: a pressure in the cavity is x;and a distance between the cathode and anode is y; and a product of xand y is approximately equal to a Paschen value of a discharge voltagebetween the cathode and the anode.
 21. The detector of claim 20, whereinthe Paschen point has a value between 25 and 75 Torr-mm.
 22. Thedetector of claim 21, wherein the Paschen point has a value between 45and 55 Torr-mm.
 23. The detector of claim 19, wherein the firstconductive layer comprises a conductive material selected from a groupconsisting of tungsten, copper, nickel, gold, silver, nickel-iron,barium oxide, cesium, hafnium, molybdenum, and the like.
 24. Thedetector of claim 23, wherein: the first conductive layer is a cathode;and the second conductive layer is an anode.
 25. The detector of claim24, wherein: the distance between the cathode and the anode is less than200 microns; and the pressure of the gas in the cavity is greater than250 Torr.
 26. The detector of claim 24, wherein the anode is a grid. 27.The detector of claim 26, wherein the cathode comprises tungsten. 28.The detector of claim 25, wherein the noble gas is neon.
 29. Thedetector of claim 28, further comprising H₂ in the cavity.
 30. Means fordetecting light comprising: means for containing a cavity; and means forproviding a discharge situated in the cavity upon receipt of light,situated in the cavity; and wherein the means for containing a cavityand the means for providing a discharge are fabricated with MEMStechnologies.
 31. The means of claim 30, wherein the cavity comprises anoble gas.
 32. The means of claim 31, wherein: the means for providing adischarge comprises a cathode and an anode; the cathode and the anodeare separated by a distance d; the cavity has a pressure p; and aproduct of d and p is approximately equal to a Paschen value of abreakdown voltage between the cathode and the anode.
 33. The means ofclaim 32, wherein: d is less than 200 microns; and p is greater than 250Torr.
 34. The means of claim 33, wherein: the cathode comprises aconductor selected from a group consisting of tungsten, copper, nickel,gold, silver, nickel-iron, barium oxide, cesium, hafnium, molybdenum,and the like; and the anode is conductive and light transparent.
 35. Themeans of claim 34, wherein the Paschen value is between 25 and 75Torr-mm.
 36. The means of claim 35, wherein the cavity comprises fusedsilica.
 37. The means of claim 36, wherein the noble gas is neon.
 38. Amethod for detecting light with a miniature sensor having a longlifetime comprising: forming a cathode on the substrate; and forming acavity with a top layer, having an anode, on the substrate; and wherein:the cavity comprises a noble gas at a pressure greater than 200 Torr;and the cathode and anode have a spacing less than 250 microns; and thecavity is formed with MEMS technology.
 39. The method of claim 38,wherein the top layer and anode permit the passage of ultra-violet lightinto the cavity.
 40. The method of claim 39, wherein a certain magnitudeof ultra-violet light in the cavity may cause an electrical dischargebetween the cathode and anode.
 41. The method of claim 40, wherein thecavity comprises: a volume between the anode and the cathode; and atrench in the substrate around the cathode.
 42. The method of claim 41,wherein; the substrate comprises fused silica; and the top layercomprises fused silica.
 43. A light sensor comprising: at least oneultra-violet light detector formed on a substrate; and wherein the atleast one ultra-violet light detector comprises: a cathode formed on thesubstrate; a trench formed in the substrate around the cathode; a spacerformed on the substrate proximate to a perimeter of the trench; and atop layer having an anode on one surface, situated on the spacer withthe anode facing the cathode, and resulting in a cavity containing thecathode and the anode.
 44. The sensor of claim 43, wherein the cavityincludes the trench.
 45. The sensor of claim 44, wherein the at leastone ultra-violet detector is a MEMS device.
 46. The sensor of claim 45,wherein the cathode and anode have a spacing between them of less than200 microns.
 47. The sensor of claim 46, wherein the cavity comprises anoble gas.
 48. The sensor of claim 45, wherein the substrate comprises aplurality of ultra-violet detectors.